Are there any good resources for finding out which hop varieties have similar aroma, flavour and bittering potential. E.g. a place to lookup a hop could be substituted for another hop for bittering for flavor and for aroma?

The list on wikipedia is a good starting point, but it's not very well cross referenced - because that is not what wikipedia is for.

I have been doing a lot of hop chemistry research and if you want a more then basic comparison check this site out
http://beerlegends.com/hops-varieties Click on a hop variety you are interested in and there will be a chart for every characteristic of a hop you could ever want to know.
Get to know your hop oil fractions (or compositions)(Myrcene Humulene Caryophyllene and Farnesene), it's the last chart at the bottom. If you don't know much about hop oils a click on the oil name next to the chart will take you to a brief description and sort the hops by that specific oil. besides charts it also has a hop substitute list and common beer styles list as well